A typical electric transfer switch (or circuit beaker) is used to switch electric loads from a normal main utility power supply to a standby emergency power supply when a power outage occurs caused by a variety of reasons, for example, earthquake, flooding, bombing, or other utility blackouts. When the power outage is over, the switch is also used to switch electric loads from the standby emergency power supply back to the normal main utility power supply.
In an electric transfer switch, one of the concerns is to control the transition time in switching from the main power supply to the standby emergency power supply. For example, it may be necessary to disconnect loads from both electric power supplies for a controllable time period to allow residual electricity to dissipate or discharge before being switched to the standby emergency power supply so that the transferred loads are appropriately switched over. Accordingly, the switch is in a neutral position for a controllable period of time, whereby the electric loads do not electrically contact either of the electric power supplies, e.g. the normal power supply and the standby emergency power supply. In other words, a programmed transition mode is provided in which the loads are disconnected with either power supply for a controlled time period.
A typical electric transfer switch is very expensive because many mechanical parts are included and complicated designs are involved. For relatively low power applications (e.g. &lt;200 Amps), a simple and low cost electric transfer switch is needed to switch electric power between a normal power supply and an emergency power supply.
In addition, during a quick make and break operation, the transfer switch opens or closes its moveable contacts with respect to its stationary contacts. After connecting the moveable contacts to the stationary contacts at one closed position, e.g., when the moveable contacts are connected to the normal position or to the emergency position, it is required to lock the transfer switch in the closed position to maintain the contact between the moveable contacts and the stationary contacts. In the closed position, when a "fault" current occurs, it is also required not only to maintain the contact between the moveable contacts and the stationary contacts, but also to lock the moveable and stationary contacts in the closed position so that the repulsive electromagnetic forces at contacts do not open the switch. Accordingly, it is desired to design an electric transfer switch which solves these problems.
U.S. Pat. No. 3,246,100 discloses a load transfer switch mechanism for opening and closing movable contacts of a circuit breaker. However, this transfer switch mechanism includes a complicated mechanical linkage, an electrical motor, and a worm and gear assembly. Further, the switch includes a spring assembly to preload a toggle mechanism so that when the spring assembly reaches over center position, the kinetic energy in the spring assembly causes a rapid change over the contact arms from one stable position to the other stable position. Accordingly, the patent discloses two stable positions and the instant switch between the two stable positions. Therefore, the patented switch does not have a controllable transition period for the switch being a neutral position which disconnects electrical loads from both power supplies to allow residual electricity to dissipate or discharge before being switched to the other power supply.
Therefore, there is a need for an electric transfer switch which overcomes all the above problems. The present invention provides a new, improved, and nonobvious electric transfer switch, which solves these and many other problems associated with existing electric transfer switches.